Add generic New Classes scenario manager

[lrzpellegrini]

Already implemented in my private codebase, working on porting it to Avalanche.

This class will allow the user to create a NC (New Classes) scenario given a couple of generic train and test Datasets.

The user will be able to create a manager instance that will be an iterable. This iterable will output the incremental "task"s or "batch"es (terminology to be defined) and will also allow the user to execute certain task/batch complex management operations.

This is very similar to the current loader being implemented in the Avalanche codebase, but will allow the user to plug in his/her own dataset. Also, due to being extremely generic, this will speed-up the integration of new datasets.

The code I've already implemented in my private codebase works fine, but is complex. I'm working on slimming it down a bit. Here is a list of already implemented features, please feel free to comment if you feel we need even more features!

Features (already implemented):

• Get current/cumulative/task-specific train/test datasets
• Variable number of tasks (or "incremental batches" for task-free scenarios)
• Allow the user to customize the number of classes in each task
• Class shuffling given a seed
• Ability to define a fixed class order (for results reproducibility)
• Remapping class original IDs to range(0, n_classes) (very useful when creating confusion matrices and for algorithms based on dynamic head expansion which require class idxs in ascending order)

Side features (already implemented):

• Wrapper Dataset class that makes any Dataset sliceable and funny-indexable
• Wrapper Dataset class that makes any Dataset transformable (like the ones in torchvision)

To be defined (even in future development phases):

• Terminology: the manager will output "task"s. That is, training/test sets made only of patterns of certain classes. The question is: is the "task" terminology "ok"? Or is too much related to task-oriented scenarios? Consider that, apart from the terminology considerations, the users will be able to use this manager both in multi-task and task-free (single incremental task) scenarios...

Feel free to comment.

Keep up with the excellent work you've been doing!

[vlomonaco]

Thanks @lrzpellegrini for this, I think we can start by adding this calss as an additional Avalanche feature, than we will see if we can use it as a parent class for all the data loaders.

As for the nomenclature, i think the word batch is more general...